Hydraulic borehole mining devices of the type with which the present invention is concerned exhibit a good potential for mining underground coal from the ground surface with a minimum disturbance of the surface itself. This general technique involves the drilling of a borehole from the ground surface to the underground coal deposit and the use of a pair of high-pressure water jets produced by oppositely directed jet nozzles to cut or otherwise provide fragmentation of the coal. A slurry jet pump is used to entrain, in a slurry, the fractured coal, i.e., the coal particles, and to transport these particles back to the ground surface.
The present invention is particularly concerned with devices of this type wherein a screen is used to cover the intake or inlet apertures through which the coal is picked up. The slurry jet pump is utilized to create a suction force in the area of these intake apertures so that the coal particles are drawn in through the screens, and entrained in the water jet created by a jet nozzle supplied with inlet water from the ground surface, to form a slurry such as referred to above. The screens are used as a filter to prevent oversized particles from plugging the jet pump and/or the slurry conduits. However, while the use of such screens provides important advantages, the use of such screens also create a problem. In particular, these screens are prone to plugging or blockage by mineral particles lodged therein or accumulated thereon. Such plugging can obviously greatly limit the mineral particles which are picked up by the device and hence impair the entire operation.